In recent years, 6-substituted aminopurines have assumed considerable biochemical significance. Some compounds of this type promote plant growth and belong to the group of growth regulators termed cytokinins (Letham, Ann. Rev. Plant. Physiol. 18, 349, 1967). Kinetin (N6-furfuryladenine) was the first molecule to be discovered with cytokinin activity. It was originally isolated from autoclaved herring sperm DNA (Miller et al. 1955, J. Am. Chem. Soc. 77:1392). Cytokinins closely related to kinetin occur also as modified bases in soluble RNA (Skoog et al., Science 154:1354, 1966). In the serine and tyrosine tRNAs of yeast, plants and animals the cytokinin is adjacent to the anticodon. The growth of mammalian cell cultures is inhibited by certain N6-substituted adenosines with cytokinin activity (Grace et al., Proc. Am. Assoc. Cancer Res. 8:23, 1967).
Cytokinins are important plant hormones regulating many aspects of plant development. Recent studies on transgenic plants with altered cytokinin metabolism or signalling revealed interesting consequences of cytokinin deficiency or disruption of cytokinin perception (Werner et al, Proc. Natl. Acad. Sci. USA 98:10487, 2001, Werner et al, Plant Cell 15:2532, 2003; Riefler et al., Plant Cell 18:40, 2006). Modulation of cytokinin levels by exogenous application or regulation of their endogenous levels genetically through cytokinin oxidase/dehydrogenase (CKX, EC 1.5.99.12), a key enzyme involved in cytokinin degradation, have already shown possible applications in agriculture. For example, exogenous application of cytokinins led to shortening of the time to anthesis in tomato (Sawhney and Shukla, Am J Bot 81:1640, 1994) or reversion of male sterility in barley (Ahokas, Proc. Natl. Acad. Sci. USA 79:7605, 1992). Anther- and pollen-specific expression of CKX in maize was shown to be a potential tool for generating male sterility for production of hybrid varieties of traditionally non-hybrid crops (Huang et al., 2003). Recent work reported CKX involvement in regulation of rice grain production (Ashikari et al., Science 309:741, 2005).
We have recently discovered that novel generations of CKX inhibitors could be based on 2-substituted 6-anilinopurines. The most promising substituents are 2-chloro, 2-fluoro and 2-amino groups in direction to specific CKX inhibitors preparation.
It is an object of this invention to provide cytokinin analogues having improved selectivity and efficiency index in the inhibition of cytokinin oxidase/dehydrogenase, i.e. that are less toxic yet more efficacious than the analogues known heretofore.